Liquids are pumped over long distance for storage and consumption. Water supplied to residents of a city is pumped from sources like lake and or rivers located far away from the city. Invariably catchment areas are located at a level lower than the consumption points. In a pumping station located close to the catchment area pumps are used to pump water over a terrain having many high and low regions. Once the flow is established, the conveying conduit and the pump casing are filled with water and water from a low lying catchment area is transferred to a storage reservoir or tank at a higher level, near the point of consumption. When the pumping unit is switched off water contained in the conduit line flows back due to gravity into the low level reservoir. This flow back establishes a siphon to drain the water from the high level tank back to the low level reservoir.
A siphon is a continuous tube that allows liquid to be drained from a reservoir through an intermediate point that is higher than the liquid level of the reservoir to a lower level. Flow of liquid in a siphon is driven by the difference in hydrostatic pressure without any need for pumping. It is necessary that the outlet end of the tube be lower than the liquid surface in the reservoir.
Liquids rise over the crest of a siphon as they are pushed by atmospheric pressure. A tube at the starting stage of a siphon is filled with liquid and atmospheric pressure acts on both ends of the conduit. The longer leg of the tube carries a greater weight of liquid. Gravity then drains the liquid through the longer leg, and this creates a low pressure inside the tube and at the other end of the tube and the liquid starts to flow into the tube establishing a siphon. Once started, a siphon requires no additional energy to keep the liquid flowing up and out of the reservoir. The siphon will pull the liquid out of the reservoir until the level falls below the intake causing air to enter the tube (cavitation/evolving of air dissolved in water) or until the outlet level of the siphon equals the level of the reservoir, whichever happens first. Capillary action can enhance the siphon and cavitation may modify the phenomenon and cause the siphon to break.
Cavitation is defined as the phenomenon of formation of vapour bubbles of a flowing liquid in a region where the pressure of the liquid falls below its vapour pressure. Cavitation often occurs in pumps, propellers and impellers.
The maximum height of the siphon crest is limited by atmospheric pressure, the density of the liquid, and its vapour pressure. When the pressure exerted by the weight of the liquid equals that of atmospheric pressure, a vacuum will form at the high point and the siphon effect will end. The liquid may boil briefly until the vacuum is filled with the liquid's vapour pressure. For water at standard atmospheric pressure, the maximum siphon height is approximately 10 m (33 feet) and for mercury it is 76 cm (30 inches).
In a conventional pumping system, non return and butterfly valves are required to arrest the reverse flow of liquid. Presence of valves and non return valves create resistance in forward flow of water and result in hydraulic losses and increases pumping cost.
Another problem encountered in large pumping system is trapped volume of gas, usually air, at the start of pumping. As there are many high and low regions in the lay out of the delivery conduit depending on the terrain, many air pockets are created at intermediate high regions within the conduit. These compressed air pockets and change in direction of water flow creates water hammer and surge creating high stresses and consequent damages. Surge conditions may occur for various reasons like pump start/stop sequences, power supply failure, and valve failure in the liquid system. Also change in demand and rapid valve operations causes surge conditions leading to water hammer.
Existing Knowledge:
Some of the devices used for prevention of back flow due to siphon effect are described herein under:
U.S. Pat. No. 6,443,181 discloses a “Backflow prevention apparatus”, consisting of a valve with a valve seat, a pivoted closing device coupled to the valve seat provided with a seal having a closure mechanism operated by a linkage mechanism. This device is not suitable for large pumping system as the presence of the valves and non return valves create resistance in forward flow of water and result in hydraulic losses and increases pumping cost.
U.S. Pat. No. 6,742,534 discloses a “Method of damping surges in a liquid system” This system includes a surge vessel in which air is trapped and in the event of a surge in the system, the volume of trapped air within the surge vessel is changed to dampen the surge by providing pressure sensors and a control apparatus for operating a compressor, air inlet valve or air relief valve to maintain constant the mass of the trapped air in the surge vessel is maintained constant, irrespective of the liquid level within the surge vessel. The method as disclosed in this document is not suitable for preventing back flow of liquid due to siphon effect from a higher level tank or reservoir to a lower level liquid source.
U.S. Pat. No. 6,792,962 discloses an “Enhanced backflow prevention apparatus and method”. This device is a plunger operated anti-siphoning device for use in a irrigation sprinkler system. The plunger is generally annular in shape and blocks water flow from the outlet channel into the inlet channel but allows flow from the inlet channel to the outlet channel. When the inlet channel is closed, the plunger permits air to flow into the outlet through the cap. This device is not suitable for large pumping system as the presence of the valves and non return valves create resistance in forward flow of water and result in hydraulic losses and increases pumping cost.
Hence there is a need for system for pumping water from a source at a relatively lower level to a reservoir at a relatively higher level and prevent back flow of liquid do to siphoning.